Please can anyone offer advice on work experience evidence.

I am self employed and have been for about 4 years.
I should be able to get work references from most of my clients.
As a self employed person what other evidence do I need to give to show I've worked for at least 20 hours per week over the past 2 years?
My income is fairly low and doesn't really support the number of hours I've actually worked. If I submit my tax return as evidence they might question my low income.

Any advice is greatly appreciated.